The invention generally relates to the field of point level monitoring. More particularly, the invention relates to automatically setting a threshold for a point level device, such as a capacitance point level device, without a user selecting whether the probe is currently covered or uncovered.
One method of measuring a level of material in a tank is by using capacitance probes mounted to the tank. A capacitance probe may be used to determine a point level, that is, whether the material level is above or below a certain point. To make such a determination, electronics may measure probe capacitance. Because the capacitance from the probe to a reference point varies depending on whether or not the probe is covered, the electronics may compare the measured capacitance to a threshold value to determine whether or not the probe is covered.
In a conventional point level probe device, the probe is typically calibrated by covering the probe with material so that the device can learn the capacitance value of the probe when covered. Then, the probe is uncovered so that the device can learn the capacitance value of the probe when uncovered. The device sets a threshold value between the two values. For example, a user may fill a tank and calibrate the level device for the covered state. Then the user may empty the tank and calibrate the level device for the uncovered state and the device determines a value between the covered and uncovered value as the threshold. This technique may be unacceptable because it may involve adding and removing large quantities of material.
Another disadvantage with this technique is that there may be a considerable time lapse between the two calibration procedures. As such, it may take an unacceptably long time to calibrate both states. For example, the device may be first calibrated for the covered state. The device, however, may not be in the uncovered state for several months, for example, at the next plant shutdown for maintenance. Even if both states of a device are initially calibrated, over time the device may not perform optimally due to changes in probe capacitance, for example, due to material residue, coating effects, and the like. Also, if the dielectric properties of the material changes, the device may require recalibration.
In cases where calibrating both states is impractical, typically the level device is first calibrated in one state (e.g., the uncovered state) and a threshold capacitance is established by adding a fixed reference value (e.g. a stored predefined reference value) to the calibration value. Recent devices may perform continual adjustment of the threshold value thereby overcoming some of the problems of conventional point level devices. Such calibrations, however, typically require a user to select whether the probe is covered or uncovered. This can lead to errors if the user makes the wrong selection.
In view of the foregoing, there is a need for a system and method for automatically adjusting a threshold value for a point level device without a user selecting whether the probe is covered or uncovered.
The invention is directed to systems and methods for automatically adjusting a threshold for a point level device without a user selecting whether the probe is covered or uncovered.
In one embodiment, a method is provided for automatically setting a threshold for a point level device comprising the steps of measuring a first value from a point level element, measuring a second value from the point level element, determining whether the point level element is covered or uncovered based on whether the second measured value has increased or decreased with respect to the first measured value, and determining a threshold value based on at least one of the first and second measured values. The method may determine whether the point level element is covered or uncovered based on whether the second measured value has increased or decreased by a predefined amount with respect to the first measured value.
In another embodiment, a point level device is provided comprising a processor that measures a first value from a point level element, measures a second value from the point level element, determines whether the point level element is covered or uncovered based on whether the second measured value has increased or decreased with respect to the first measured value, and determines a threshold value based on at least one of the first and second measured values.
The above-listed features, as well as other features, of the invention will be more fully set forth hereinafter.